A filter element for filtering oils and other liquids should remove a maximum amount of the particulate material while offering minimum resistance to flow of the liquid and should have adequate capacity to provide a useful life of service. A relatively coarse filtering media permits readily flow of the liquid through the element but serves to remove only the coarser particles. On the other hand, the use of a relatively fine filtering media results in the removal of both fine and coarse particles, but the useful life of such an element is unduly limited by clogging of the surface and consequent resistance to liquid flow. In the past, filtering elements have been utilized which included both coarse and fine filtering media in series. With a filtering element of this type, the liquid is caused to flow in series, first through the coarse filtering media and then through the fine filtering media, so that the coarse particles are filtered out in the coarse media and the finer particles, that pass through the coarse media, are caught in the final fine media.
Oils and other liquid hydrocarbons vary in viscosity as the temperature varies. This presents a serious problem in filters for use in lubrication systems of internal combustion engines. A filtering element which is designed to operate effectively in the removal of particles when the oil is warm, and flows readily through the medium, offers so much resistance to the flow of cold viscous oil that it is necessary to by-pass the filter when the oil is cold to insure a continuing supply of lubricating oil. While this by-pass method is utilized to avoid damage to the bearings and to provide an adequate oil supply, it does involve an undesirable compromise in that unfiltered oil carrying potentially damaging particles is supplied to the bearings and other moving parts of the engine.
In an attempt to overcome this problem, filters have been designed in which coarse and fine filtering media are arranged in parallel with respect to the flow of oil, so that a portion of the oil will pass through the coarse media at all temperatures which results in the removal of the coarser particles. However, some of the finer particles in the oil are not removed and are delivered to the bearings and other moving parts.
U.S. Pat. No. 3,384,241 is directed to a graduated density liquid filter element in which fine and coarse filtering media are arranged such that the resistance to flow to the liquid through the media varies at different locations along the length of the filter element so that the element is capable of providing adequate flow, as required by the bearings, at all times. In accordance with the invention of U.S. Pat. No. 3,384,241, the graduated density filter element has a cylindrical shape and is composed of a pair of concentric layers of graduated thickness of coarse and fine filtering media with the coarse media disposed upstream of the fine media. With the construction of the aforementioned patent, the pattern of flow shifts with respect to the length of the element as flow resistance is increased due to loading at particular locations. The shifting of the flow pattern provides an automatic adjustment which results in a gradual increase in pressure drop across the element and increased service life. Problems, however, have been encountered in the fabrication of the graduated density filter element of the aforementioned patent in that it has been difficult to obtain a controlled variable or graduated thickness for the two layers. In practice, a cylindrical fabric sock is wrapped around the central tube of the filter and reversed to provide an annular space. The operator simultaneously introduces both coarse and fine filtering media into the annular space and the material is compacted by a ram. With this procedure, the operator is required to manually vary the amounts of coarse and fine material as they are introduced into the annular space and to maintain each media in a separate strata or layer in an attempt to provide the progressively varying thickness for the layers. This procedure is not only difficult to control, but is time consuming and costly.